That's because the RAM is shared with the GPU and most of these games would require a GPU with at least 2-4GB on top of the normal system requirement to have at least 8GB. So, 8GB of RAM would be cutting it close on a mac since part of that would have to be sacrificed for the GPU.
They are running emulated games in their own separate virtual machine, because Intel games expect a 4k page size and the OS is running with a 16k page size.
Virtual Machines require their own chunk if memory overhead, so the resource usage can't help being higher than a native MacOS game's would be.